A basic problem presently facing utility companies is variation in demand for power. Electric utilities typically incorporate (1) base load generators, (2) intermediate load generators, and (3) peak load generators to accomodate daily load demands. Base load generators, which operate 100% of the time at constant throttle, are powered by relatively less expensive fossil fuel or by nuclear generators. About 70% of the system power output is generated by the base load generators operating at lowest delivered cost. The intermediate load generators, which are typically gas turbines or diesel engines, are powered by less efficient fossil-fueled steam generators and gas turbines. The intermediate load generators are normally run only during the day, and furnish about 25% of the power at significantly higher costs. The remaining power is derived from peak load generators requiring costly and scarce fuels. Peak load generators are operated only on demand and serve the immediate load peaks of the day.
In the past, electric utilities have attempted to reduce the high cost of peak power generation through pumped hydroelectric storage. Typically, water is pumped uphill during off-peak hours and stored in a reservoir for later peak hour use. This method is practical only for large scale use and is limited to locations where geography permits construction of large reservoirs. In another approach, air is compressed during off-peak hours, and the potential energy stored in the compressed air is later released for use during peak hours. This has also been shown to be of limited practicality.
It has been proposed to store energy during low demand periods by using electric power to rotate a massive flywheel, and tapping energy from the flywheel during high demand periods. A flywheel rotor in conjunction with an electric motor/generator unit is provided, wherein by operating the unit as a motor, the flywheel rotor is brought up to operating speed and allowed to coast. Energy is extracted from the flywheel rotor during coasting by operating the motor/generator unit as a generator.
Rotating flywheel systems have not been entirely successful due to low operating efficiency caused by, among other factors, losses from eddy currents, friction, windage and relatively poor energy storage per unit weight of the flywheel. In addition, there is a substantial hazard of catastrophic failure of the rotor.
In U.S. Pat. No. 3,158,750 to Roes, a mechanical energy storage device includes a rotor rotatably supported on a frame. A series of magnets are spaced apart from each other along the outer rim of the rotor. A synchronous armature winding mounted on the frame is disposed so as to intersect the magnetic field established by the rotor magnets and to cause torque to be applied to the rotor when alternating current is passed through the armature winding. The rotor is supported by spokes extending to a shaft that in turn is journaled to a set of conventional magnetic bearings.
Although generally somewhat satisfactory for limited use in an outer space environment, the efficiency of the Roes system is insufficient for storage of energy of the magnitudes required by electric utilities. The magnets contained on the rotor in Roes create magnetic fields that tend to induce eddy currents in the stationary, metallic frame members in proximity to the rotor. These eddy currents result in eddy current losses that substantially diminish storage efficiency. The magnets along the rim of the rotor, being discrete elements, also create balancing problems and induce additional stresses on the rotor thereby increasing the chances of failure. The magnets themselves are difficult to manufacture because they must be accurately ground to size and imbedded into the rotor so as to create minimum imbalance and stress concentrations.
Accordingly, an object of the present invention is to provide a new and improved mechanical energy storage device.
Another object of the present invention is to provide a new and improved energy storage system incorporating a flywheel rotor.
Another object of the present invention is to provide a flywheel energy storage device that is highly efficient and less prone to structural failure.
Yet another object of the present invention is to provide a motor-driven, flywheel-type energy storage device including a continuous, spokeless, flywheel rotor having no discrete rotor elements.
Still another object of the present invention is to provide a motor-driven, flywheel-type energy storage device, wherein eddy current losses are minimized.
Still another object of the present invention is to provide a motor-driven, flywheel-type energy storage device, wherein a flywheel rotor is devoid of permanent magnets so as to reduce eddy current losses by interaction of a moving magnetic field with stationary elements.
Yet another object of the present invention is to provide a motor-driven, flywheel-type energy storage device, including a flywheel rotor having high specific strength per unit weight density.
Still another object of the present invention is to provide a new and improved motor-driven, flywheel-type energy storage device, wherein a flywheel rotor is magnetically levitated to eliminate mechanical drag between the rotor and stationary elements.